The biotic composition of desert soils

The introduction of culture-dependent and -independent methods have recently shed light on the composition of microbial communities associated with desert plants. Recent data has demonstrated that deserts are usually dominated by heterotrophic Actinobacteria, Proteobacteria, and Chloroflexi (all bacterial organisms). Heterotrophic organisms require organic compounds produced elsewhere as their nutritional support. In the harsh desert environments, heterotrophs may face extreme starvation for their preferred organic energy and carbon sources. Thus, some of these organisms have evolved to reversibly enter a metabolically less active state termed dormancy during periods of environmental pressures. The state of dormancy increases cellular resistance to external stresses while reducing energy expenditure.

Interesting: Desert soil microbiota appear to take advantage of brief “water pulses”, such as occasional precipitation, condensation of dew or fog, and ice or snow melts (in polar deserts) to generate biomass and accumulate reserve compounds in preparation for long periods of water scarcity. In the desert environment, water is commonly provided in the form of early-morning dew, which is followed by desiccation as temperatures rise and relative humidity declines during the day. Some biocrust microbes must therefore rapidly respond to a dew event by activating respiration and photosynthesis for biomass production and then rapidly shut these systems off. This is known as the “energy reserve hypothesis”. Another hypothesis recently put forward (but not mutually exclusive), the “continual energy harvesting hypothesis”, suggests that heterotrophic microbes in desert ecosystems possess hidden metabolic flexibility. They may meet energy demands during starvation by continually harvesting atmospheric trace gases (lithoheterotrophy) or sunlight (photoheterotrophy) as alternative energy sources.